If the situation is grave, you need to contact the county animal regulation authority. Every rescue in Oregon is full-up to the rafters, we personally started fostering this year due to the lack of space.
Are the people willing to surrender their animals, and that's why they need a rescue? Maybe surrendered as individual horses, I could find a foster home for one or two. If they haven't agreed to surrender their horses, unfortunately, and the horses are truly victims of abuse or neglect, you need to get the law involved first. Horse rescues can do nothing until the horses are seized by the county (except put pressure on tha animal regulation authority). If the situation is bad enough to remove the horses, people can proceed from there. If it's not serious enough to warrant removal, there's nothing anyone can do but offer to buy the horses outright.
Let me know what the status is, and I will contact who I can.